The flat display device has many advantages such as thin profile, electricity economization, no radiation and so on, and has been widely used. The existing flat display device mainly includes a liquid crystal display device (LCD) and an organic light emitting display (OLED). The thin film transistors (TFTs) are important components of flat display devices, may be formed on glass substrates or plastic substrates, and typically used as switching devices and driving devices of LCDs and OLEDs, for example.
In recent years, the display technology has been rapidly developed, and the thin film transistor technology has developed from the original amorphous silicon (a-Si) thin film transistor to the low temperature polysilicon (LTPS) thin film transistor. The LTPS thin film transistors have many advantages, and for example, the LTPS thin film transistors have a higher electron mobility, which not only can effectively reduce the area of thin film transistors and improve the aperture rate, but also can reduce overall power consumption while improving the display brightness. For another example, the higher electron mobility may allow a part of the drive circuit to be integrated on the substrate, reduce a number of the drive integrated circuit IC, greatly enhance the reliability of the display panel, and significantly reduce manufacturing cost. Therefore, LTPS thin film transistor has gradually become a research hot topic in display field.
The structure of the existing LTPS thin film transistors mainly includes a base substrate and a polysilicon active layer, a gate insulating layer, a gate electrode, a source electrode and a drain electrode successively provided on the base substrate, and the source electrode and the drain electrode are electrically connected to the polysilicon active layer through a via hole. The gate insulating layer covering the polysilicon active layer is formed by a deposition process, and a defect density of the gate insulating layer is relatively large and thus carriers in the polysilicon active layer are easy to diffuse into the gate insulating layer resulting a formation of larger leakage current and an increase in the instability of the electric performance of LTPS thin film transistors. Therefore, the prior art still needs to be improved and developed.